In the past, barley breeding has been performed by employing traditional crossing techniques. Specifically, individual varieties having different genotypes are crossed, desirable lines are selected from the obtained progeny lines based on cultivation characteristics such as agricultural characteristics, environmental adaptation, and disease resistance, and the selected lines are bred to produce genetically fixed barleys. This process needs five to six years normally, and in addition, it also requires to select only the barley variety from the genetically fixed barleys that has excellent fermentation characteristics. In order for a preferable barley for a malt alcoholic beverage to be selected, an enormous amount of time is required to be spent.
Such barley breeding requires breeders to carry it out based on their experiences. To select the barley that is believed to possess excellent fermentation characteristics, breeding needs to be done by subjective judgments of experienced breeders.
The barleys used as materials for malt alcoholic beverages (such as beer and low-malt beer) contain carbohydrases such as α-amylase and β-amylase. Carbohydrates in barley seeds are decomposed to low molecular weight sugars by the action of these carbohydrases during malting or saccharization.
In WO 99/00514 the present inventors have reported on the correlation between the property of β-amylase and barley varieties that the final apparent attenuation which represents the fraction of the portion in carbohydrates in the barley malt to be utilized in fermentation excels as the thermostability of β-amylase increases. However, with respect to the therostabilities of β-amylases for various barley varieties classified into four types, even the one with most excellent thermostability only displays a residual activity of 45 to 50% upon heat treatment at 57.5° C. for 30 minutes. No thermally stable varieties of cultivated barleys that surpass said variety have been discovered.
Eglinton et al. discovered a line in wild barleys (H. Spontaneum) for which the thermostability of β-amylase was high, but its utilization in breeding involved difficulties because it is a wild species. (Eglington, J. K., P. Langridge and D. E. Evans (1998) Thermostability variation in alleles of barley beta-amylase. J. Cereal Science 28: 301-309.)